1. Field of the Invention
The present invention relates to a method for determining recording laser power for a super-resolution optical recording medium from which recording marks can be reproduced that are smaller than the resolution limit of a reproduction optical system. The invention also relates to an apparatus for recording information on such a super-resolution optical recording medium.
2. Description of the Related Art
Recently, as described in, for example, Japanese Patent Laid-Open Publication No. 2003-6872, a super-resolution optical recording medium has been suggested which enables the reproduction of a recording mark train that is smaller than the diffraction limit of a reproduction optical system.
This super-resolution optical recording medium enables the reproduction of a minute recording mark (also known as a super-resolution recording mark) that is smaller than the resolution limit of a reproduction optical system, however, the principle of how this is enabled has not yet been made known.
Generally, in a reproduction system employing an optical system with a reproduction wavelength of λ and an objective lens numerical aperture of NA, a cut-off frequency is defined as fc=2 NA/λ, and a period corresponding to a frequency higher than that cut-off frequency cannot be read. In terms of length, in a reproduction system employing an optical system with a reproduction wavelength of λ and an objective lens numerical aperture of NA, a periodic structure having a period less than Dc=1/fc=λ/NA/2 cannot be recognized as a period. The value Dc is referred to as the diffraction limit. The period in optical recording is defined by a paired mark (or pit) and space. Super-resolution reproduction is a reproduction technique capable of separating a paired mark and space in a periodic structure having a period shorter than Dc. When the length of a mark is equal to the length of a space, the diffraction limit corresponding to the above period can be conveniently represented by the length of the mark (or the space), Lc=Dc/2=λ/NA/4. The length Lc is referred to as the resolution limit.
A general method for determining recording laser power, when data is recorded on an optical recording medium, is described in, for example, Japanese Patent Laid-Open Publication No. Hei 3-91124. In this case, a plurality of data sets is test-written at different laser powers. The written data sets are reproduced, and error characteristics are measured to determine a recordable range. Then, the central value of the recordable range is used as the recording laser power.
In Japanese Patent Laid-Open Publication No. Hei 3-91124, the recordable range is a range in which errors in reproduction data can be corrected when a laser power within that range is used for recording.